Attaching this for those who use the Metro for travel and for all tourists too.

Phase III - Completion by 2016

Out of 2 new lines and 10 route extensions proposed for Phase III, cabinet approvals have been obtained for 2 new lines and 5 line extensions totaling 140 km, with an estimated cost of 350 billion (US$6.37 billion).[66] Construction has already begun on many of these, and by mid-2012, tenders of all approved lines will be awarded. These approved lines are:

In addition, the following corridors are expected to be cleared by the government in the days to come:
Yamuna Vihar – Shiv Vihar : 2.77 km (2 stations) feasibility study to be conducted
Rithala to Bawana: 12 kms, DPR under preparation
Mundka to Bahadurgarh: 11.2 kms, DPR submitted in 2007. Fresh discussions to take place between Delhi and Haryana governments.
Dwarka to Najafgarh: 5.5 kms long corridor, the case is being processed by DMRC for further approvals.
If all these proposed corridors are approved as part of Phase 3, Delhi Metro will have a network of about 340 kilometres after phase 3.

Phase IV

Phase IV has a 2021 deadline, and tentatively includes further extensions to Sonia Vihar, Burari, Mukundpur, Reola Khanpur, Palam, Najafgarh, Narela, Ghazipur, Noida sector 62, extensions of Violet line, Green line, Line 8, having a total length of over 100 km. There might be some changes in plan before actual construction starts on these lines.